HookManager used keyboard_hook before assignment

When destroyed, HookManager's __del__ method calls UnhookKeyboard which
was checking for a boolean keyboard_hook, but this variable would not
have been set unless there was a previous call to HookKeyboard.  The
__init__ of HookManager was mistakenly initializing key_hook instead of
keyboard_hook.
Merge pull request #1 from dpfrey/master

HookManager used keyboard_hook before assignment